The present invention relates to a novel process for the preparation of 2,2',4,4',6,6'-hexanitrobibenzyl (HNB). This compound can be converted by various procedures to 2,2',4,4',6,6'-hexanitrostilbene, an important thermally-stable explosive which is also useful as a nucleating agent promoting a desired mode of crystallization of trinitrotoluene.
Prior to the present invention, the preferred method for preparing HNB was described in U.S. Pat. No. 3,505,413 and in a paper by Shipp and Kaplan, Journal of Organic Chemistry, 31, 857 (1966). The references teach dissolving TNT in one part tetrahydrofuran and two parts methanol by volume. A solution of sodium hypochlorite, which is made alkaline by the addition of sodium hydroxide, is then slowly added to the solution of TNT. During the addition of the sodium hypochlorite to the solution of TNT, the temperature did not exceed 35.degree. C. After a suitable aging period (15 minutes), the crystalline material was filtered and then washed with methanol and dried. It is reported that the amount of HNB obtained is 79% of theoretical.
The prior art processes, while effective, do have a decided disadvantage in requiring the use of large volumes of solvent mixtures, which include expensive compounds. The U.S. Pat. No. 3,505,413 explains that the solvent should be one which will dissolve TNT at ambient temperatures and below and which, with the addition of a lower aliphatic alcohol, will provide a homogenous solution with an alkaline metal hypochlorite. Examples of such solvents are tetrahydrofuran, p-dioxane, diglyme and acetonitrile, which are all water-miscible materials.
Also, in accordance with the prior art process, the temperature at which the reaction proceeds must be kept low in order to minimize undesired side reactions between TNT and alkali as well as the competing reaction of the solvent system. The Shipp et al. paper suggests temperatures under 15.degree. C.
In my co-pending application filed Sept. 26, 1979 Ser. No. 79,128, directed to the preparation of HNB, a solvent system is disclosed which utilizes a water-immiscible solvent and an aliphatic alcohol. While this system has a decided advantage over the Shipp, Kaplan process, it still requires a dual solvent with additional expense and handling.